Effect of a 90? Elbow on the Accuracy of an Insertion Flowmeter, Results and Comparisons for 4 and 6 in. Diameter PVC Pipe

摘要

Thermal energy consumption in buildings withchilled or hot water distribution systems is oftenmonitored through the use of some type of flowmetering device. These flowmeters can be fixed types,such as venturis or orifices, or insertion flowmeterswhich can be more easily installed and removed. Theeasy removal and reinstallation of the insertion typeflowmeters makes them good choices for use inexisting buildings or in retrofit projects. Besides theinstallation benefits, insertion flowmeters can also beinstalled while the pipe is in service or ''hot tapped".With any type flowmeter however, location in the pipeis a critical problem and deserves specialconsideration. Ideally, the meter should be inserted inexisting pipe with a minimum of 10 to 15 diameters ofstraight pipe upstream of the meter location. This israrely the case in existing piping distribution systems.It is much more common to be faced with only one ortwo candidate metering locations and these often arevery short straight runs or will have elbows upstreamand downstream of the proposed metering location.This paper reports on flow measurement errorresulting from an insertion flowmeter installeddownstream of a 90? elbow. The measurement errorswere compared for tests conducted in 4.0 and 6.0 inch(0.1 and 0.15 meter) diameter PVC pipe. Theinsertion flowmeter was a nonmagnetic, tangentialpaddle wheel type. The flowmeter was located from 2to 10 pipe diameters downstream fiom a 90? elbowwith fluid velocities ranging from 1.0 to 10.0 ft/s (0.3to 3.0 m/s). At each flowmeter location, the meter wasrotated in 45? increments around the circumference ofthe pipe to quantify the effect of circumferentiallocation on flow error.The flowmeters were tested at the energy meteringcalibration facility at the Texas A&M UniversityEnergy Systems Laboratory Riverside campus.Flowmeter output was compared to mass flowmeasurements obtained 6om precision load cellsmounted beneath a 1342 ft^3 (38 m^3 ) weigh tank. Alloutput is given in terms of percent error relative to theload cells. Final results are presented as a bction offlowmeter downstream location, circumferentialrotation angle, and fluid velocity. Circumferentialmeter location was found to be a very important factor.The percent difference for the tested flow metersranged 6om -23% to -5% in the 4.0 in. (0.1 m) pipeand 6om -33% to 1% in the 6.0 in. (0.15 m) pipe. The''best" location for these flowmeters was at zerodegrees rotation angle, regardless of pipe size or meterlocation relative to the upstream 90? elbow.